Sintered austenitic-ferritic chromium-nickel steel alloy

ABSTRACT

Powdered austenitic chromium-nickel stainless steel is blended with a powdered metal ferrite stabilizer, such as molybdenum, and the resulting blend is sintered to produce a new steel, namely, an unwrought austenitic-ferritic chromium-nickel alloy having, as sintered, desirably high tensile and yield strengths and other desirable properties.

United States Patent [72] Inventor Theodore R. Bergstrom Little Canada, Minn.

[21] Appl. No. 743,588

[22] Filed July 10, 1968 [45] Patented Nov. 16, 1971 [73] Assignee Minnesota Mining and Manufacturing Company St. Paul, Minn.

[54] SINTERED AUSTENITIC-FERRITIC CHROMIUM- NICKEL STEEL ALLOY 9 Claims, No Drawings [52] U.S. Cl 29/182, 75/ l 28 [51] Int. Cl B22f 3/00 Primary Examiner-Benjamin R. Padgett Assistant Examiner-R. L. Tate Attorney-Kinney, Alexander, Sell, Steldt & Delahunt ABSTRACT: Powdered austenitic chromium-nickel stainless steel is blended with a powdered metal ferrite stabilizer, such as molybdenum, and the resulting blend is sintered to produce a new steel, namely, an unwrought austenitic-ferritic chromium-nickel alloy having, as sintered, desirably high tensile and yield strengths and other desirable properties.

SINTERED AUSTENITIC-FERRITIC CI-IROMIUM- NICKEL STEEL ALLOY FIELD OF THE INVENTION This invention relates to powder metallurgy of stainless steel. In another aspect, it relates to the manufacture of sintered stainless steel articles from powdered metals. In another object, it relates to a novel mixture of powdered stainless steel and powdered metal ferrite stabilizers. In a further aspect, it relates to a shaped article of a novel chromium-nickel steel alloy and a method for its preparation using principles of powder metallurgy.

BACKGROUND OF THE PRIOR ART Austenitic chromium-nickel stainless steels are enjoying an increasing wide-spread industrial application as engineering alloys because of their resistance to corrosion and desirable mechanical properties. Unfortunately, these steels are not significantly hardenable by or responsive to heat treatment; phase transformation is suppressed by the nickel constituent in these steels and austenite (the gamma form of iron) is substantially retained on cooling from the gamma region. The mechanical properties of these austenitic steels cannot be controlled or varied by the usual type of heat treatment, such as quenching and tempering. Changes in mechanical properties, such as strength, are brought about only by expensive, timeconsuming cold working (rolling) and annealing, resulting in the so-called wrought austenitic stainless steels. (See Forming of Austenitic Chromium-Nickel Stainless Steels 2nd Ed. (1954), published by The International Nickel Co., Inc., New York, N.Y.).

In accordance with this invention, desirable changes in austenitic chromium-nickel stainless steels are brought about by a certain novel powder metallurgy technique, resulting in a sintered or unwrought alloy having increased strength and other desirable mechanical properties.

Powder metallurgy broadly is not a new type of metallurgical process but it is receiving increasing application in the manufacture of metallic articles, extending as it does the design limits of liquid metallurgy. An excellent description of this metallurgical process is found in Review of the Powder Metallurgy Process, July, 1966, published by the U.S. Army Production Equipment Agency, Manufacturing Technology Division, Rock Island Arsenal, Ill.

Powder metallurgy has been used to make metal articles approaching the physical properties, such as density and strength, of cast or wrought alloys of similar composition. In fact, powder metallurgy has been applied to stainless steel see Progress in Powder Metallurgy," Vol. 16, pp. 120l29 (1960, Capital City Press, Montpelier, Va.). Although useful stainless steel articles have been made by the powder metallurgy technique generally high-compacting pressures and prolonged sintering at elevated temperatures have been found necessary in order to produce high density articles. Generally stainless steel articles commercially produced by powder metallurgy procedures have densities of 80 90 percent of theoretical density and interconnected porosity. These densities are not as high as desired and result in mechanical properties that are not as good as those of annealed wrought articles of similar composition, and the interconnected porosity increases their susceptibility to corrosion. Further, in order to obtain desired strengths, it generally has been necessary to coin or mechanically work the sintered stainless steel articles.

By further way of background of the prior art, mention should be made of U.S. Pat. No. 2,593,943 (Wainer) which discloses molding mixtures of metal powders with a heat-fugi tive binder, the metal powders employed therein including powdered molybdenum, nickel, cobalt, and other metals, as well as mixtures of both a metal and an alloy. However, there is no teaching in this patent of sintering a powdered metal mixture of austenitic chromium-nickel stainless steel and exclusively a ferrite stabilizer such as molybdenum to form an austenitic-ferritic chromium nickel alloy of increased strength. U.S. Pat. No. 2,792,302 (MOTT) discloses making sintered articles from 18-8 stainless steel using 10 to 15 weight percent of a binder which can contain a relatively small amount of molybdenum disulfide as a die lubricant, which, upon subsequently being reduced during the sintering operation, has an insignificant effect, if any, on the properties of the sintered article. U.S. Pat. No. 3,223,523 (Adler) discloses a powder metallurgy technique in which stainless steel powder (A151 302) is blended with an aqueous solution of a salt of molybdenum, copper, or nickel, such as ammonium molyb' denate, which salt is reduced to form a metallic coating on the stainless steel powder, the amount of metallic coating being sufficient to improve the green strength of the powder compact and apparently less than that which would increase the strength of the resulting sintered article or change the finished properties thereof.

BRIEF DESCRIPTION OF THE INVENTION Briefly, this invention provides a new alloy or steel, characterized as an unwrought chromium-nickel steel alloy, by sintering a mixture of powdered austenitic chromium-nickel stainless steel and powdered metal ferrite stabilizer, such as molybdenum, to form during sintering an austenitic-ferritic structure. This new alloy has a number of desirable properties and can be made in accordance with this invention with densities ranging from those which are relatively low to those which approach theoretical density, with strengths equaling or surpassing that of cast or wrought and annealed stainless steels of substantially the same elemental composition. These objects can be achieved by powder metallurgy techniques without resorting to specially produced powders, very high densification pressures and extremely high temperature sintering cycles of prolonged duration, or subsequent mechanical working and annealing operations. The low density products of this invention have particular utility as filter elements and the highly dense articles can be used, for example, in fabricating complex shapes that ordinarily would be cast, forged, or machined.

DETAILED DESCRIPTION OF THE INVENTION The stainless steel powders used in this invention are commonly known in the art as austenitic chromium-nickel stainless steels, these alloys generally containing 16.0 to 26.0 weight percent chromium, 6.0 to 22.0 weight percent nickel, 0.03 to 0.25 weight percent (max.) carbon, and occasionally some other elements added to develop certain specific properties, such as 1.75 to 4.00 weight percent molybdenum or small amounts of titanium, tantalum, and niobium to minimize formation of chromium carbides, especially in welding. Standard types of these steels have been assigned numbers and specifications by the American lron and Steel Institute. These are generally known in the art as stainless steels of the A181 300 series, types 301, 302, 304, and 305 generally referred to as l8-8f" stainless steel, and the workhorse type 316 generally referred to as l 8-8-Mo." All of these AlSl stainless steels of the 300 series are applicable in the practice of this invention. However, AISI 303 and 304 are preferred over grades such as AIS! 316 and 317 because a greater amount of the ferrite stabilizer can be used without producing sintered articles with reduced ductility.

Powdered AISl stainless steels of the 300 series are commercially available in various grades or sizes and can be prepared by the atomization of molten metal. Generally, the powdered stainless steel used in this invention will have a mesh size of -50. In making highly dense articles, I prefer to use -325 mesh and in making less dense or porous articles I prefer to use S0+325 mesh, such as I200+325, -+200, 50+100, or blends therof, suitably selected to produce the desired micronic rating or bubble point, and. to that end, small amounts, e.g.. 12() Weight percent, of 325 mesh can be blended with the coarser powder, i.e., the 50+325 mesh. (The term mesh" referred to herein means mesh size according to U.S. Standard Sieve.) Preferably, in the practice of this invention, the stainless steel powder is used in its alloyed form, sometimes referred to as being a prealloy"; however, it is within the scope of this invention to use blends of the powdered individual metal elements in the same proportions found in the prealloyed steels since the amounts of the elemental metal constituents in the sintered articles will be equal to those found in prealloyed stainless steel.

The metal ferrite stabilizers used in this invention (in combination with the powdered austenitic chromium-nickel stainless steel) are a known class of materials, most of them having body centered, cubic crystal form. They are distinguished from the austenite stabilizers, such as nickel and cobalt, which do not produce the desired results, such as high density, when used in a similar fashion in the practice of this invention, even when used in fine particle size (1.2-3 microns) at a level of 6 weight percent. The ferrite stabilizers used in this invention include molybdenum, titanium, vanadium, tungsten, chromium, zirconium, silicon, tantalum, and niobium. It is also within the scope of this invention to use combinations in the form of mixtures or alloys of two or more of these stabilizers, such as a molybdenum-tungsten combination, or a molybdenumvanadium combination. The amount of stabilizer used will depend upon the particular stabilizer to be used and the properties desired in the subsequently sintered article. Generally, the amount of stabilizer blended with the powdered stainless steel will be that amount, functionally expressed, sufficient to impart a desirably high tensile or yield strength without undesirably imparting brittleness to the article. The particular stabilizer to be used and the amount thereof can be determined by those skilled in the art in possession of this disclosure by simple routine tests involving correlating various levels of stabilizer with the mechanical properties of the corresponding sintered articles. Generally, the amount of stabilizer used will amount to l to 11 weight percent, based on the total weight of the blend of powdered stainless steel and powdered ferrite stabilizer. Generally, low amounts do not impart the desired increase in strength and fast sintering rate, and high amounts will result in brittleness of the sintered article. In the case of the stronger ferrite stabilizers, such as silicon and zirconium, amounts of l to 3 weight percent may be sufficient to achieve the desired results. In the case of chromium, tungsten, titanium, and vanadium, these stabilizers are preferably used in amounts of 3 to 9 weight percent, and molybdenum is preferably used in amounts of to 7 weight percent.

The mesh of the powdered ferrite stabilizer can vary from relatively coarse to relatively fine, but fine mesh of 325 is preferred because of the greater distribution of the resulting ferrite in the grain boundaries. The size of the ferrite stabilizer powder is preferably expressed in terms of the Fisher Standard Subsieve Series. Generally, powdered ferrite stabilizer having Fisher Numbers in the range 0.5 to 44 microns will be applicable, though that in the range of 2 to 10 microns is preferred. it is also within the scope of this invention to use reducible oxides, hdyrides, and, less desirably, salts of such ferrite stabil' izers, since such precursors will be reduced during sintering to the metal. Such salts include the nitrates, sulfates acetates, halides such as chlorides and bromides, and the like, as well as ammonium molybdenate.

The blended mixture of the austenitic stainless steel powder with the powdered ferrite stabilizer can be deposited in the form of a loose powder on a suitable substrate or in a suitable mold, as in the case of slip casting, and sintered to form a rigid sintered article. Alternatively, the blended powdered mixture can be compacted or pressed to form a shaped article which is then sintered.

Where organic heat-fugitive binders are used to form a shaped article, binders of the nature disclosed in U.S. Pat. Nos. 2,593,943, 2,709,651, and 2,902,363 can be employed, such as methylcellulose. Various solvents can be used in conjunction with these binders, such as water, as well as various plasticizers, such as glycerin. Useful die lubricants which can also be used include stearic acid, and zinc, calcium, and lithium stearates. Where organic materials or adjuncts are blended with the mixture of powdered stainless steel and ferrite stabilizer, the resulting shaped articles can be dried or slowly heated prior to sintering, or even partially sintered, e.g., at l050-l 200 C., in a reducing atmosphere, in order to volatize, burnoff, and/or decompose the organic material, taking suitable precautions to minimize any carbon from being left in the sintered article.

The blending of powdered stainless steel, powdered ferrite stabilizers, and binders and other various adjuncts where used, can be carried out in a conventional manner in various types of commercially available mixers, tumblers, blenders, rotating drums, and the like, care being taken to insure that the blend is homogeneous and the components well dispersed. Where a binder is used, the blend will be in the nature ofa plastic mass, dough, or clay, and can be shaped and dried, for example, on a rolling mill or by means of extrusion, injection molding, etc. The shaped article can then be compacted under pressure, if desired, before sintering.

Compacting of the blended powdered mixture, either as a dry mixture or as a dough, or even after partially sintering a dough to burn-off organic binder, can also be carried out in a conventional manner, using either hot or cold pressing, such as die pressing, isostatic pressing, etc., the compacting pressures range from 4,000 to 200,000 p.s.i. Actually, the high compacting pressures normally used in compacting powdered stainless steel will not be required in the practice of this invention in order to obtain desirably high strength and density in the sintered article, and thus the longevity of die parts, etc., will be far greater, with the attendant cost savings. In fact, the desired objects of this invention can be readily achieved with low compacting pressures in the range of 20,000 to 50,000 p.s.i.

Shrinkage of the shaped articles upon sintering will occur, as it does in conventional powder metallurgy, and this should be compensated for by making the article to be sintered with oversize dimensions, etc. Generally, linear shrinkage will be I to 25 percent.

The sintering step of this invention will be generally carried out at sufficiently high temperatures and have sufficient duration to achieve at least during the sintering step the austeniteferrite structure and the desired increased tensile strength in the sintered article. Generally, the sintering temperature will be below that at which any melting of the metal powders occur. Sintering temperatures useful in the practice of this invention will be generally in the range of l200 to [400 C,, and preferably from 1250" to 1350 C., this latter preferred temperature range being the range where the ferrite phase is readily formed. The duration of sintering will vary and can be from 10 minutes to 2 or 3 hours or longer. In any event, the sintering temperature will be sufficiently high and of sufficient duration to cause the formation of two-phase austenite-ferrite microstructure. The sintering operation is carried out in a conventional reducing atmosphere or under vacuum or in an inert gas such as argon. The reducing atmospheres particularly useful include hydrogen and anhydrous or cracked ammonia, the dew points of these gases being 40 F. or lower. The sintering furnaces which can be used include the conventional resistance or induction heated gastight shell or muffle furnace of the pusher, hump, or batch types. After sintering, the sintered articles are preferably rapidly cooled through the region where the ferrite phase is partially unstable, so as to minimize rejection of ferrite formers and maintain the ferrite formed during the sintering operation. The rate of cooling necessary to retain the ferrite formed during sintering can be determined empirically by simple routine cooling tests by those skilled in the art. Means for effecting the rapid cooling necessary to preserve the ferrite phase, or at least 50 to volume percent of that formed during sintering, are available in the art, such cooling being carried out by quenching sintered articles from their sintering temperatures in cooled furnace gas, other gas such as argon or nitrogen, air, water, oil, or the like. Slow cooling, such as furnace cooling, of the sintered articles can be employed but generally is not preferred since this favors the formation of sigma and related undesired phases, which impart brittleness and other generally undesirable properties to the sintered article. However, slow cooling can be used in those instances wherein the presence of these normally deleterious phases is desired or is of no consequence.

The microstructure of the preferred sintered articles of this invention is substantially two-phased: austenite and ferrite. Other phases, namely sigma and/or related phases, may be present if the sintered article is slowly cooled as described above. The austenite-ferrite two-phase structures can be heated further at higher sintering temperatures very near the melting point of the structure to form structures which are substantially all ferritic, or, by appropriate heat treatment, the twophase structures can be transformed into substantially all austenitic structure. These essentially single-phase structures, however, will revert to the two-phase austenite-ferrite structure if reheated to temperatures, e.g., l250-l 350 C., favoring their coexistence. In any event, inrorderto produce sintered articles having desired properties, such as strength and density, it is essential in this invention to sinter at the temperature where the two-phase structure exists, regardless of whether it is destroyed or retained by further heating at higher or lower temperatures or upon cooling. That is, these desirable properties are not dependent on the existence of the twophase structure in the cooled, sintered article, but are dependent on a sintering step where such two-phase structure is formed. However, in the case of single phase ferritic structures, these will exhibit reduced ductility and toughness and reduced corrosion resistance to salt solution, i.e., sea water.

Generally, at sintering temperatures favoring such twophase structure or at ambient temperatures where such twophase structure is retained, the ferrite phase will comprise 4 to 80 volume percent, preferably to'60 volume percent, and the balance will be substantially austenite.

The presence of the ferrite phase results in a faster rate of sintering due to the increased diffusion rate of this phase and it imparts magnetism to the sintered article. The grains of austenite and ferrite are randomly distributed and the grain size of these phases in the sintered article is relatively fine, e.g., 5-8 according to ASTM El9-33, and is in contrast to the relatively coarse grain of prior art sintered stainless steel caused by the costly long high sintering temperatures necessary to obtain dense articles.

X-ray studies of quenched sintered articles of this invention show, for example, face centered, cubic (FCC) diffraction lines, attributed to austenite, with a relative intensity of about 100, and body centered cubic (BCC) diffraction lines, attributed to ferrite, with a relative intensity of 70. Slow or furnace cooling of such sintered articles showed FCC relative line intensity of 100 and BCC relative line intensity of20, and two very weak lines attributed to sigma phase.

Rapidly cooled specimens when viewed under an optical microscope show microstructure characterized as grains of austenite dispersed in an essentially continuous matrix that was ferrite during sintering but has transformed at least partially to a very fine mixture of austenite and ferrite during cooling from the sintering temperature. Where a relatively large amount, e.g. 9 percent, of the molybdenum stabilizer is used, the grains of austenite will be needlelike or lenticular, and where a relatively small amount of molybdenum (e.g., 3 percent) is used, the grains of austenite will be irregular equiaxed in shape. Low-temperature sintering, e.g., 1,250C., tends to reduce the amount of ferrite and produces irregular equiaxed austenite grains; at high sintering temperatures, e.g., l,350 C., the amount of austenite decreases and the grains of austenite appear lenticular or needlelike.

Most importantly, the, tensile strength of the sintered article will be significantly greater than that obtained by sintering powdered stainless steel of the A181 300 series in the absence of ferrite stabilizers. Dense sintered articles of this invention made with -325 mesh austenitic stainless steel powder (and ferrite stabilizer) will have as sintered ASTM E8-66 tensile strengths as high as 55,00080,000 p.s.i., and even as high as 110,000 p.s.i., these values being as much as 25 to 200 percent greater than those obtained by sintering stainless steel powder without ferrite stabilizer addition. The sintered articles of this invention also have very high yield strengths, a property of considerable importance to structural designers. Yield strength is usually defined as the stress required to impart a permanent deformation of 0.2 percent in the article. Dense sintered articles of this invention prepared from -325 mesh 'austenitic stainless steel powder (and ferrite stabilizer) will have ASTM E8-66 yield strengths as sintered as high as 25,000 to 80,000 p.s.i., which values are 50-400 percent higher than that of as sintered stainless steel articles of similar composition produced by powder metallurgy (without ferrite stabilizer). These high yield strengths even substantially exceed that obtained by annealed wrought stainless steel of similar composition,

The apparent density of the dense sintered articles will also be significantly greater (e.g., 5-25 percent greater) and generally will be in a range of to 95 percent of the theoretical density (as sintered), as determined by mercury porousimetry described by the American Instrument Co. in its Bulletin 2300 1960).

In the case of the porous sintered. articles of this invention, the as sintered tensile strength and absolute micronic rating values of the sintered articles can be multiplied to obtain a product value which is useful as a parameter for evaluating the mechanical properties of the articles without reference to the particle size of the stainless steel powder used in preparing them. For example, a tensile strength of 17,475 psi. multiplied by an absolute 'niicronic rating of 14 microns, gives a product value or parameter of 244,650. Theparameter values of the porous articles of this invention will be as high as 200,000 to 500,000 and as much as to 450 percent higher than that of porous articles made of sintered stainless steel powder without ferrite stabilizer addition.

Where reference is made to as sintered values, this means the value of the article after sintering and cooling to room temperature and prior to any subsequent or post treatment, such as mechanical working and annealing.

As far as known, the desirably high strengths and/or densities of the articles of this invention can be obtained in the prior art onlyby repeatedly cold working and annealing stainless steel of the casttype obtained by liquid metallurgy, or by prior art powder metallurgy techniques involving significantly greater compacting or pressing pressures and long high temperatures sintering and subjection of the sintered article to subsequent repeated cold working and annealing.

The novel alloy ofthis invention can be used in manufacturing articles of either a relatively low density or porous nature, which would be particularly suitable where the sintered articles are used as filter elements, or relatively dense articles having densities approaching theoretical densities. Such high densities are particularly suitable in the fabrication of such articles as die pressedor injection molded parts, such as a cam, valve housing, etc., seamless tubing for heat exchangers and immersion heaters, corrugated recuperative or regenerative heat exchangers (made without welding or brazing). The dense'shapecl articles can also be used for architectural applications such as window casings and decorative railing supports, burner grids of corrugated or foamed structure, acoustic materials made as a foamed structure, catalyst carrier and catalyst support structures, dinnerware, etc. Dense sintered articles of this invention can be made highly impervious, for example, by injection molding, such articles being advantageously employed in applications where leakage or corrosion would present problems if relatively porous sintered stainless steel were used. It is also within the scope of this invention to subject the sintered articles to finishing operations which result in even denser articles or better mechanical properties,-such operations including, for example, coining and resintering. However, the as sintered articles in most cases will have the properties desired and further processing will be unnecessary though useful in some cases to achieve final dimensional tolerances.

EXAMPLES The objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples. as well as other conditions and details, should not be construed to unduly limit this invention.

EXAMPLE 1 Two batches of powdered metal were made using 325 mesh prealloyed powdered stainless steel of the AIS] 316L type, one batch being made in accordance with this invention using 3 weight percent powdered molybdenum with a Fisher Number of 3.27 microns. In each batch, 100 g. of the powdered material were mixed with 3 g. of methyl cellulose (4,000 c.p.s.) in a twin shell blender for 1 hr. and then for about 20 min. in a sigma blade mixer with 9.5 cc. of l6.6 weight percent solution of glycerin in distilled water until a stiff clay or plastic mass was produced. The wet clay was then rolled to a 0.060 inch thick sheet on a rubber mill with a roll speed ratio of 1.4:1. The sheets were cut into specimens linch X2 inches, placed in a vacuum drying oven and dried at ll5 F. The specimens were then pressed at 20,000 p.s.i. and sintered in a vacuum furnace by heating from room temperature to 350 C. in about 3 hrs. and then heating to l325 C. and holding for 8 hrs., the samples being suspended in l00 mesh alumina during sintering and furnace cooled (734 C./min. over l3006 00 C.). in table I below, the in table of these two runs are shown. Metallographic mounts of the sintered specimens were etched with ferric chloride. In estimating the amounts of the phases, light-appearing grains were considered austenite and the relatively darker matrix was considered as mixed grains of austenite (light) and ferrite (dark) which was considered all ferrite at sintering temperature.

One thousand g. of 325 mesh A181 3 16L powder, 30 g. of molybdenum of a 2.4 micron Fisher Number. and g. of methylcellulose (4000 cps) were dry blended for 1 hr. To this was added l 10 cc. of 10 percent aqueous solution of glycerin in distilled water. The batch was mixed to a claylike consisteney in a sigma blade mixer and the batch was rolled into sheet and pressed. Blanks were cut from the sheet and these were sintered in a hydrogen atmosphere for 4 hrs. at l3l5 C. The specimens were water quenched from l,300 C. and mechanical tests were performed. The carbon content after sintering averaged 0.06 weight percent. Table II summarized the results.

1 Strengths were determined according to ASTM 158-66, using pin loaded specimens 1 guage length and $4 width.

2 Values shown for each run are averages of 3 specimens.

6 vol. percent ferrite.

These data show that the use of ferrite stabilizer results in a sintered article having a significantly greater density and causes the formation of a two-phase microstructure of austenite and ferrite.

EXAMPLE 2 In this example, a series of six runs was made in which compacted mixtures of stainless steel of A151 303 type and varying amounts of molybdenum (Fisher Number 3.27 microns) were sintered and furnace cooled. The compacting sintering, and

The data of table lll show that high strengths and densities can be obtained with desirable elongation. The values shown for these properties are considerably better than what is now commercially achieved in powder metallurgy and compare very well with many wrought austenitic stainless steels in the annealed condition. The closeness of the apparent and real density values shows that the connected porosity in the specimens IS very minor.

EXAMPLE 4 in this example, a series of runs was made in which compacts of 325 mesh AlSl 304L or MS] 316L stainless steels dry blended with varying amounts ofmolybdenum (Fisher No. 4.2 microns) were pressed at 20,000 p.s.i. and sintered at various temperatures for various periods of time in dry hydrogen and rapidly cooled (ll0 C./min.) and the properties of the green compacts and sintered articles determined and compared. For purpose of comparison, other runs were made in which stainless steel compacts made without molybdenum addition were prepared and sintered under these varying conditions. Results are summarized in table lV,

TABLE IV Run 1 1 2 3 4 5 6 7 8 Stainless steel used. 304L 304L 304L 304L 30% 304L 316L 316113 Amount of M in blend, wt. percent. 0 3 ti 9 0 0 intering:

Time, hrs 2 2 2 1. 25 1. 25 2 2 p, 1, 250 1, 250 1, 250 1, 250 1, 300 1, 300 1, 350 1, 350 Densities:

Green, g./cc....... 5. 00 5.14 5.17 5. 28 5. 08 5. 20 5. 56 5. 59 Smtered, g./cc 6. 33 6. 84 7. 38 7.50 6. 57 7. 58 0. 72 7. 43 Percent of theoretical 79. 4 85.0 01.0 01. 8 82. 4 03. 4 84. 2 92. 3 Strengths: 2

0.2% offset yield, K s.i 19. 4 29, 6 37. 9 21. 7 58. 5 17. 9 30.0 Ultimate K S.i 33. 5 57.0 81.0 86.0 50. 4 95. 6 43. 3 64. 4 Hardness, Vickers diamond pyramide 3 68 122 130 271 80 216 69 119 Elongation, percent 20. 3 7. 0 2. 0 25.9 19.5 25.0 16. 5 Phases Austenite, V01. percent 100 78 60 95. 6 6O 96 90 Ferrite vol. percent 0 22 40 80 4.4 40 4 10 Grain size of phases: 5

Austenlte 6 6 7 8 6 8 5 5 Ferrite 8 8 7 8 8 8 8 8 Value shown for each run is an average of 3-6 specimens. Specimens for tensile strength tests were made with a Heller DL- following ASTM 138-66 and MPIF 1053.

3 Using 10 kg. load.

4 Microstructure evaluations were made as described in Example 1.

AESTM 1519-33 (smallest size on chart is 8).

The data of table IV show again the increase in density and strengths obtained through the use of ferrite stabilizer. In addition, the data show that in general these values increase with increasing molybdenum content, though elongation of the sintered article falls off with higher amounts of molybdenum ad- In this example, a series of runs was made in which various 35 results are summanzed ferrite stabilizers were blended with AlSl 304L stainless steel,

1001 die and rested on an Instron machine,

The powdered materials were blended in a twin shell blender for about 30 min. 'Those sintered articles whose tensile strengths were determined were made from compacts prepared by pressing the blended powders in a Haller DL-lOOl die in accordance with ASTM 158-66 and MPIF 1063, these specimens having been pressed at 20 tsi before sintering. All sintering was performed at sintering temperatures of l300 C for 1.25 hrs. (except in Runs 3, l2, and I3, where sintering was at [350 C. for 2 hrs). in a palladiumsilver purified hydrogen atmosphere using induction heating. The sintered articles were rapidly cooled in the furnace. The

le Vl.

TABLE \'1 Density, percent of theoretical Strength (K s.i.) Ferrite stabilizcr(s) Green Sinterod 0.2% used and amt. thereof, (1:0.5 ($1.0 oil'sct wt. percent or blend max.) max.) yield Ultinmtn 63. 6 82. 4 .21. 7 50. 4 64. 1 03. 7 55. 2 9s. 0 04. U 115. I) 53. 4 105. 0 68. 2 04. 8 45. 3 82. 7 (ll. 4 113. 5 78. 2 JG. 8 (13.0 81;. a 40. u 78. 8 63. 0 04. u 45. 1' s3. 2 63.1 as. 2 43. o 00. 5 3%Zrl1 63.1 04.2 3% M0+1.5% H2... 63.8 113.2 30.1 40.3 88.6 3% Mo+1.5% 11112.... 63. u so. 0 0. 3 44. 0 70. 4 6% N 64. 2 85. 5 36. T 1'17 53. 0 13 6% C0 (i4. 4 85. 8 3T. 8 21. fl 67. J

the blends compacted, sintered, and rapidly cooled. The staln- 5 5 The above data show the applicability of a host of ferrite staless steel powder had a mesh size of -325. All of the stabilizers were used in elemental powder form except for the titanium, vanadium, and zirconium stabilizers, which were used as hydrides. For purposes of comparison, runs were also made with nickel and cobalt additions. The sizes of the ferrite stabilizers used are shown in table V TABLE V Powder Size Mo 2-4 microns Cr 3 microns W 0.9 microns TiH, 6-9 microns ZrH, 2-8 microns Co 1.2 microns Ni 3-5 microns VH, 325 mesh" Si 325 mesh bilizers in the practice of this invention as well as combinations thereof, and also show that Ni and Co, by comparison, are inferior.

EXAMPLE 6 Following the procedure of example 5, a compact was made and sintered from a blend of elemental metals used in amounts matching the composition ofAlSl 3041.. In one run, the blend contained 6 weight percent of powdered molybdenum and the compact prepared from this blend had a green density of 70 percent that of the theoretical density, the sintered density of this compact being 87.4 percent of theoretical. The compact made from the blend without molybdenum addition had a comparable green density of 70.9 percent of theoretical but by contrast the sintered density of this compact was only 81.5 percent of theoretical. The sintered article made with molybdenum had an 0.2 percent yield of 47.1 KSl, an ultimate strength of 56.1 KS1, and an elongation of 2.5 percent. The sintered article made without molybdenum had an 0.2 percent In place of methylcellulose, other binders can be used, such as polypropylene, polystyrene, stearin mixed with vegetable oil, etc.

In preparing Batch 1, the stainless steel and methycellulose were blended in a sigma blade mixer for about 30 min. and then the glycerin-water solvent was added. The material was mixed into a claylike consistency under a vacuum of about 29 The data of table VIl show that the addition of molybdenum significantly aided the sintering process and allowed the production of dense impermeable parts by injection molding. Heretofore it has been necessary to cast or machine stainless steel bar to obtain dense complex parts. The subject invention now makes it possible to produce complex, dense. strong parts in stainless steel by injection molding without resorting to the more expensive process of machining or the process of molten metal casting.

EXAMPLE 8 Six batches of various commercial grades of stainless steel powder were prepared, some of which were blended with powdered molybdenum (Fisher Number 4.2 microns).

Two hundred g. of each batch were blended with l g. of methylcellulose and then with 35-45 cc. of percent aqueous solution of glycerin, and each batch was converted into a claylike material. This material was then rolled using a rubber mill to produce sheet. The green sheet was sintered in hydrogen by heating from room temperature to 1,350" C. in 12 hrs. and holding at 1,350" C. for 2 hrs., and furnace cooled. The resulting sintered sheets were cut into suitab le specimens inches of mercury. The clayhke material was added to a for flow, bubble point, and tensile testing. These runs and the Frobring Mini-Jector in ection molding machine (Model results obtained are summarized in table Vlll.

TAB LE V111 Runs Stainl ss steel used 316L 316L 316L 310L 316T, 316L M0811 50+100 50+100 0l0+200 100+200 200+335 -200+325 Amount of M0 in blend, wt. percent. 0 5 0 5 0 Pressure (em. 1120) to produce following air flows through specimen:

37C.1.11 2.2 3,3 4.7 12.3 33.2 -1 74 0.1.11 5. l) 7. ll 13. 0 28. 7 71. 3 397 111 e.l'.h. .1. 3 12. .1 20. 5 45. 0 100 730 148 e.f.h 13.7 18.7 31 00.8 143.0 Strengths: 1

0.2% ofiset yield, p.s.i 4, 250 1, 400 10,075 4, 466 1., 400 Ultimate, p.s.i 000 5, 050 l. 825 11, 200 (l, 325 17, 475 Apparent density 30 46 4t 54 70 Bubble point (A 1 req. to burst b bble,) 2 cm. 1120.. 8. 4 8. 0 13. 8 1G. 2 30. 8 40. 3 Absolute micronic r g, microns" 72 70 42 38 20 14 Parameter 3 64, 800 353, 500 78, 473 4'35, 600 1'16, 500 31-1, 650

1 Specimens for tensile strength were made from flat 1 gauge length and tested (as in Ex. 3) at room. temperature using a loading rate of 0.5 cm ./min. alues shown are averages of 4 runs.

2 Specimens were tested for bubble point according to M IL-F-BBIEB, except that specimens were not rotated, and the absolute micronic rating was determined therefrom by dividing the bubble point into the eonversion factor of 605.

3 Parameter is the product of ultimate strength multiplied by absolute mieromie rating.

70VC100) and the clay was injected in a flat bar configuration. Batch 2 was handled in an identical manner except that the molybdenum addition was blended with the stainless steel using a V-shell blender and an intensifier rod prior to adding the powder to the sigma blade mixer. The injection molded bars of both batches were weighted and measured in the green or injected state. The green density of Batch 1 was 4.189 g./cc.

TABLE VII Apparent Apparent Percent of Carbon density, porosity, theoretical content. Batch N 0. g./ec. percent density wt. percent 1 Apparent porosity is based on volume change exhibited by specimen when subjected to mercury pressure of 3,000 p.s.i.g. (see Bulletin 2300, Amer. Inst. 00.).

The significance of the data of table Vlll can be further demonstrated by plotting the values for the various properties of the sintered articles made with and without molybdenum addition. For example, if the strengths of the sintered specimens are plotted as a function of the absolute micronic rating using a simple linear plot and linear extrapolation between data points, the plot will show that a porous membrane with an absolute micronic rating of 40 microns would be 320 percent stronger if molybdenum addition is used in accordance with this invention. A plot of strength versus density will show that, for any given density, the structure made in accordance with this invention, using molybdenum addition, will be stronger. For example, at a density of 48 percent of theoretical, the structure made with molybdenum addition is 42 percent stronger.

EXAMPLE 9 Two 3,000 g. batches of A181 3 l 6L +200 mesh) were prepared in a manner like example 8, one of these batches containing 5 weight percent molybdenum (Fisher Number 4.2 microns). The batches of clay were extruded to form 4-foot long tubes with an outside diameter of 0.504 inches and an inside diameter of 0.314 inches. The green tubes were sintered in dry hydrogen for 3 hrs. at 1,150" C. to bumoff the binder and partially sinter the tubes. The tubes were then isostatically pressed and resintered at l,350 C. for 2 hrs. in dry hydrogen. The tubes were furnace cooled to form porous tube useful as filter elements. The tubes were machined to form tensile specimens and the ultimate tensile strengths were determined. These runs and results are summarized in table IX.

TABLE IX Density of Ultimate Amount of Isostatic sintered tensile Mo blended compaction tube, perstrength with 316L pressure, cent of of sintered Run wt. percent p.s.i. theoretical tube, p.s 1 l U 40, 000 66. l 11. 900 2 5 40, 000 71. 7 24, 000

lclaim: l. A substantially two-phased, unwrought, sintered, austenitic-ferritic, chromium-nickel stainless steel alloy having at temperatures of 1250 to l,350 C. an essentially twophase structure comprising 4 to 80 volume percent ferrite phase of chromium-nickel stainless steel and the balance being substantially an austenite phase of austenitic chromiumnickel stainless steel, 50 to 95 volume percent of said ferrite phase being maintained in said alloy at room temperature 2. Alloy according to claim 1 wherein said austenite at l,250 to l,350 C. is present in the amount of 90 to 40 volume percent and said ferrite at l,250 to 1,350 C is present in the amount of 10 to 60 volume percent.

3. Alloy according to claim 1, wherein said stainless steel of said ferrite phase further comprises a metal selected from the group consisting of molybdenum, titanium, vanadium, tungsten, chromium, zirconium, silicon, tantalum, and mixtures thereof.

4. Alloy according to claim 3, wherein said metal is 3 to l 1 weight percent ofsaid alloy.

5. Alloy according to claim 3, wherein said metal is molybdenum and it amounts to 5 to 7 weight percent of said alloy.

6. Alloy according to claim 1 having as sintered" an ultimate tensile strength of 55,000 to I 10,000 p.s.i., 0.2 percent offset yield strength of 25,000 to 80,000 psi.

7. A permeable shaped article made of the alloy of claim 1, said article having as sintered" tensile strength and absolute micronic rating values whose product is in the range of 200,000 to 500,000.

8. A relatively dense article made of the alloy of claim 6 and having an apparent density of to 99 percent of the theoretical density.

9. Alloy according to claim 1 comprising 16.0 to 26.0 weight percent chromium. 6.0 to 22.0 weight percent nickel, and up to 0.03 to 0.25 weight percent carbon, based on the weight of said alloy. 

2. Alloy according to claim 1 wherein said austenite at 1,250* to 1,350* C. is present in the amount of 90 to 40 volume percent and said ferrite at 1,250* to 1,350* C. is present in the amount of 10 to 60 volume percent.
 3. Alloy according to claim 1, wherein said stainless steel of said ferrite phase further comprises a metal selected from the group consisting of molybdenum, titanium, vanadium, tungsten, chromium, zirconium, silicon, tantalum, and mixtures thereof.
 4. Alloy according to claim 3, wherein said metal is 3 to 11 weight percent of said alloy.
 5. Alloy according to claim 3, wherein said metal is molybdenum and it amounts to 5 to 7 weight percent of said alloy.
 6. Alloy according to claim 1 having ''''as sintered'''' an ultimate tensile strength of 55,000 to 110,000 p.s.i., 0.2 percent offset yield strength of 25,000 to 80,000 p.s.i.
 7. A permeable shaped article made of the alloy of claim 1, said article having ''''as sintered'''' tensile strength and absolute micronic rating values whose product is in the range of 200,000 to 500,000.
 8. A relatively dense article made of the alloy of claim 6 and having an apparent density of 85 to 99 percent of the theoretical density.
 9. Alloy according to claim 1 comprising 16.0 to 26.0 weight percent chromium, 6.0 to 22.0 weight percent nickel, and up to 0.03 to 0.25 weight percent carboN, based on the weight of said alloy. 